The present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus suitable for forming images on both sides of a transfer material through an electrophotographic method. An image forming apparatus of the present invention can be used for a copying machine, a printer and a facsimile machine.
In the conventional double-sided image forming apparatus, there has been used a method wherein an image for one side of a document formed on an image forming body is transferred onto a transfer material to be fixed which is then stored temporarily in an intermediate tray, and the transfer material is then fed out of the intermediate tray in synchronization with an image for the other side of the document formed on the image forming body to be transferred and fixed on the other side of the transfer material.
In the conventional image forming apparatus described above, the transfer material which has been subjected to fixing is fed to an intermediate tray, and the transfer material passes through a fixing unit twice. Therefore, the image forming speed is slow and conveyance of the transfer material is less reliable and often results in jams.
For overcoming this weak point, JAPANESE TOKKOSHO Nos. 49-37538 and 54-28740, and JAPANESE TOKKAIHEI No. 4-214576 have disclosed a double-sided image forming apparatus wherein after the first image is formed on an image forming body, the image is transferred temporarily onto an intermediate transfer body and the second image is formed on the image forming body. Then the first image on the intermediate transfer body is transferred onto one side of the transfer material, and the second image on the image forming body is transferred onto the other side of the transfer material. These images are then fixed simultaneously so that images are formed on both sides of the transfer material. The inventors of the present invention have studied how to put the aforesaid double-sided image forming system to practical use, and have filed applications for several further inventions, beginning with JAPANESE TOKKAIHEI No. 9-258492.
During the inventors' study, it was learned that, toner soil was deposited in the vicinity of an intermediate transfer body in an image forming apparatus equipped with an intermediate transfer body, and it was found that there is a problem which needs to be solved. The toner soil will be explained with reference to FIG. 1. Transfer unit CO which is composed of a corona charging unit and transfers toner images from an intermediate transfer body to a transfer material is arranged to be close to intermediate transfer body M through a narrow clearance as shown in the drawing, and a current of air caused by the circulating movement of the intermediate transfer body during transfer of the toner images is generated in the narrow clearance. It was observed that this current of air caused a vortex of air current R.sub.1 and a vortex of air current R.sub.2 in the vicinity of the transfer unit CO, as shown in the FIG. 1. As a result, toner sticking and toner adherence to shielding member H of the transfer unit CO were caused as shown by W.sub.1 and W.sub.2, thereby deteriorating efficiency of the transfer unit.
In the aforesaid double-sided image forming apparatus equipped with an intermediate transfer body, guiding members such as separation claws and spurred wheels are used to separate the transfer material carrying on its both sides unfixed toner images from the intermediate transfer body and to guide it to a fixing unit, and it was found that toner sticking was caused also on a guiding portion having the guiding members.
The above described problem is caused because unfixed toner is held on both sides of a transfer material at the position where toner images are transferred from an intermediate transfer body to a transfer material, and toner tending to scatter is concentrated in this portion.